Conversion of a semiconductor wafer into a semiconductor chip comprises a plurality of sequential operations of which more than 90% occur on the wafer. The entire production process may involve hundreds of operations, such as application of metals and dielectrics, doping with legating impurities, heat treatment, application of thin coatings by sputtering, chemical and physical vapor deposition, electroless deposition, etc., various types of plasma surface treatment, multiple photolithographic processes for the formations of patterns, selective removal of parts of coatings in the formation of the aforementioned patterns, etc. In a production process, a semiconductor wafer normally contains hundreds or thousands of individual semiconductor chips produced simultaneously on a single wafer, so that, upon completion of the processes on the wafer, it is cut into hundreds or thousands of individual semiconductor chips. Lead wires are then soldered or otherwise connected to the output terminals of the chips, whereby a product, which is well known to the general public as a “semiconductor chip”, is obtained.
Practically all known chips, starting from simple primitive chips with several parts and finishing with VLSIC's (Very Large-Scale Integrated Circuits) are produced in accordance with the aforementioned sequential multiple-stage scenario.
Similar situation, that involves sequential multiple-stage operations, takes place in the production of laser diodes and light-emitting diodes with the only difference that the substrates are made from modifications of GaAs and InP instead of silicon required for the chips. Another field with the same ideology is the production of flat-panel displays. Therefore, it should be understood that though the description given below will relate to semiconductor wafers, it is equally applicable to other products, such as light-emitting diodes, laser diodes, flat-panel displays, or the like.
It is understood that not all of the finally produced semiconductor chips are perfect after the manufacture and that a certain percentage of rejection always exists. The failure may occur practically on any of hundreds or thousands of the aforementioned operations involved in the manufacturing process, and therefore it is very important to control the product quality at different manufacturing stages. It has been known heretofore to provide various manufacturing processes with so-called control charts that comprise graphical representation of variations in selected parameters over time. Such parameters may comprise electrical resistance, capacitance, thickness of the coating measured by ellipsometry, reflectivity, etc. It is understood that if a certain parameter is controlled in real time, deviations of this parameter from the norm can also be corrected in real time. For example, U.S. Pat. No. 6,473,664 issued to Lee, et al. in 2002 discloses a manufacturing process automation system using a file server and its control method. In the proposed automation system, a plurality of machines is connected to a file server via a network, and the job result data produced by the machines are shared by the file server. The job result data processed from a machine (for example, a tester) are stored in the file server. Another machine (for example, a repairer) can execute a job by using the above job result data.
It should be taken into account, however, that even if all the controlled parameters are maintained within the allowable tolerances, the occurrence of the failure is not completely excluded. Such defects may be caused either by deviations of parameters, which are not controlled, or by synergistically caused unfavorable conditions that may occur in the production processes. Furthermore, the parameters are measured by a plurality of strictly specialized devices intended for measuring a specific characteristic such as resistance, film thickness, etc.
Normally, the controlled data is stored either in a central processing unit or in a database. However, the applicants are not aware of any simple, inexpensive, customer-friendly, and well-organized system for continuous recording of various parameters with a single generalized or universal sensor device in the form a certain relative value.
On the other hand, U.S. patent application No. 434,625 filed by the same applicants on May 12, 2003 shows a measurement apparatus based on the use of RST principles, wherein the apparatus has a spindle for rotatingly installing a disk, e.g., a semiconductor wafer, and a cantilever beam attached to the apparatus housing for supporting a carriage with a resonance sensor for radially displacing the sensor above the surface of the wafer. As mentioned in the above application, the rotary and radial movements of the sensor are required merely for positioning the sensor to any measurement point on the surface of the wafer. The application does not teach the use of rotation and radial movements for any other purposes. The apparatus is intended for discrete measurement of characteristics and thickness of thin films and coatings in selected points on the surface of the object being measured.
Since the present invention is based on the use of RST, a new technology developed by Multimetrixs, CA, in the beginning of 1999, it would be advantageous, for better understanding the RST principles, to shortly describe the structure and operation of a conventional inductive sensor, which is one of the electrical-type sensors, widely used for measuring, e.g., film thickness. One example of an inductive sensor of the aforementioned type is the one disclosed in U.S. Pat. No. 6,593,738 issued on Jul. 15, 2003 to Boris Kesil, et al. This patent describes the apparatus and method for thin film diagnostics and includes an example of the setup design for precision measurements using conventional (inductive, Eddy current) and capacitive sensors.
The apparatus consists of an inductive sensor and a proximity sensor, which are rigidly interconnected though a piezo-actuator used for displacements of the inductive sensor with respect to the surface of the object being measured. Based on the results of the operation of the proximity sensor, the inductive sensor is maintained at a constant distance from the controlled surface. Variations in the thickness of the coating film and in the distance between the inductive sensor and the coating film change the current in the inductive coil of the sensor. The inductive sensor is calibrated so that, for a predetermined object with a predetermined metal coating and thickness of the coating, variations in the amplitude of the inductive sensor current reflect fluctuations in the thickness of the coating. The distinguishing feature of the invention resides in the actuating mechanism of microdisplacements and in the measurement and control units that realize interconnection between the proximity sensor and the inductive sensor via the actuating mechanism. The actuating mechanism is a piezo actuator. Measurement of the film thickness in the submicron range becomes possible due to highly accurate dynamic stabilization of the aforementioned distance between the inductive sensor and the object. According to one embodiment, the distance is controlled optically with the use of a miniature interferometer, which is rigidly connected to the inductive sensor. According to another embodiment, the distance is controlled with the use of a capacitance sensor, which is also rigidly connected to the inductive sensor.
A disadvantage of the sensor of the aforementioned patent is that it is very sensitive to variations in the distance between the sensor and the film. This requirement dictates the use of expensive and complicated distance-measurement and distance control means such as micro interferometers or microscopes and piezoactuators.
U.S. patent application No. 359,378 filed by Boris Kesil, et al. on Feb. 7, 2003 describes the principles of RST which are based on the following features: 1) in contrast to the majority of known inductive sensors, the RST sensors operate at resonance conditions; 2) there exist several resonance conditions, and the RST sensors operate mainly under conditions of complete resonance; 3) under conditions of complete resonance, the Q-factor of the system “sensor-object” may be significantly higher than the Q-factor of a single inductive sensor. Incorporation of the aforementioned three features into the structure of the measurement system results in significant improvement of sensitivity and repeatability of measurements and makes it possible to measure characteristics of the film in a wide range of thicknesses from hundreds Angstroms to several tens of microns.
The new apparatus disclosed in U.S. patent application No. 359,378 consists of an inductive coil having specific parameters, an external AC generator operating on frequencies, e.g., from 50 MHz to 2.5 GHz, preferably from 100 MHz to 200 MHz, and a measuring instrument, such as an oscilloscope, voltmeter, etc., for measuring output of the sensor. The coil has miniature dimensions. The invention is based on the principle that the inductive coil of the sensor, active resistance of the coil winding, capacitance of the inductive coil (or a separate capacitor built into the sensor's circuit), and the aforementioned AC generator form an oscillating circuit in which electromagnetic oscillations are excited by the aforementioned AC generator.
The main distinction of the sensor of the device proposed in the aforementioned patent application from all conventional devices of this type is that it operates on very high resonance frequencies in comparison with frequencies used in devices described in the patent applications mentioned above, preferably within the range of 100 to 200 MHz. In order to maintain the aforementioned high frequency range, the oscillating circuit should have specific values of inductance L (several nano-Henries) and capacitance C (several pico-Farades), and in order to provide accurate measurements, the Q-factor for the above frequencies should exceed 10. It has also been found that on such frequencies the capacitive coupling between the coil of the oscillating circuit and the virtual coil induced in the film acquires the same weight as the mutual inductance between the both coils. In other words, the system can be described in terms of inductive-capacitive interaction between the sensor and the film to be measured. The capacitive coupling component determines new relationships between the parameters of the film, mainly the film thickness, and parameters of the resonance oscillating circuit. By measuring the parameters of the resonance oscillating circuit, it becomes possible to measure film thickness in wide ranges including those below 500 Angstroms, as well as other characteristics of the film.
However, in the apparatus of U.S. patent application No. 359,378, the method and system for stabilization of the distance between the sensor and the surface of the film being measured remain the same as in first-mentioned U.S. Pat. No. 6,593,738, and this feature limits significant potentials of the new method and system.
The method and apparatus aimed at still further improvement of properties disclosed in aforementioned U.S. patent application No. 359,378 are described in U.S. patent application Ser. No. 10/386,648 filed by the same applicants (Boris Kesil, et al.) as the previous application on Mar. 13, 2003. This new apparatus allows highly accurate and efficient contactless measurement of film thicknesses below 1000 Angstroms by means of a microwave resonance sensor. The apparatus consists of a special resonator unit in the form of an open-bottom cylinder, which is connected to a microwave swept frequency source via a decoupler and a matching unit installed in a waveguide line that connects the resonator unit with the microwave source.
The microwave generator is fed from a power supply unit through a frequency modulator that may sweep the frequency of microwaves generated by the microwave generator. All the controls can be observed with the use of a display, such as, e.g., a monitor of a personal computer, which may be connected to the microwaveguide line, e.g., via a directed branched waveguide line for directing waves reflected from the resonator, via a reflected wave detector, an amplifier, synchronous detector, A/D converter, and a digital voltmeter. A feedback line is going from a direct wave detector, which is installed in a line branched from the microwaveguide between the decoupler and the matching unit, to the power supply unit. The operation resonance frequency of the resonator sensor unit should be somewhere within the range of swept frequencies of the microwave generator.
In operation, the microwave source generates electromagnetic waves in a certain sweeping range that induces oscillations on the resonance frequency with a Q-factor on the order of 104 or higher in the resonator sensor unit. A distinguishing feature of the resonator of this system is that the design parameters of the resonator unit allow achieving the aforementioned high Q-factor without physical contact of the sensor unit with the film to be tested. As the surface of the film to be measured constitutes the inner surface of the resonator unit, even a slightest deviation in conductivity will exert a significant influence on the Q-factor. The Q-factor, in turn, defines the height of the resonance peak. As the conductivity directly related to the film thickness, it is understood that measurement of the film thickness is reduced to measurement of the resonance peak amplitudes. This means that super-high accuracy inherent in measurement of the resonance peaks is directly applicable to the measurement of the film thickness or film thickness deviations.
However, since this resonator is a three-dimensional or a special device, the measurement surface may have the minimum value on the order of several square millimeters. In such a construction, the diameter of the probe practically cannot be reduced beyond the limit of a few square millimeters. In other words, even though the microwave resonance sensor of the type described in U.S. patent application Ser. No. 10/386,648 is extremely accurate with regard to stabilization of the sensor-object distance, it has limitations with regard to the lateral measurement accuracy. U.S. patent application No. . . . filed by the same applicants on . . . discloses an apparatus for measuring characteristics and thickness of films and thin coatings comprising a portable hand-held sensor unit that comprises a support base for placement onto an object to be measured, e.g., a thin-film coating on a substrate, a sensor head with a resonance sensor based on resonance sensor technology, and means for adjusting the position of the sensor head relative to the object for achieving resonance conditions most optimal for measuring the characteristics and thickness, and a sensor signal receiving and processing unit having means for receiving modulated carrier signals of a predetermined frequency, e.g., 2.4 GHz. The apparatus also contains an external source of the modulated carrier signals located remotely from the portable hand-held sensor unit and having a signal transmitter unit and a predetermined frequency-receiving network with a receiving/transmitting antenna for receiving the aforementioned signals of 2.4 GHz frequency. The apparatus is suitable for measuring characteristics and thickness of films and coatings directly on objects in selected measurement points with accuracy suitable for use in the semiconductor production. It is particularly suitable for measuring properties and thickness of coatings on large objects that cannot be installed on conventional measurement stations or stationary measurement apparatuses.
However, neither the last mentioned apparatuses nor any other apparatus or method known to the applicants are suitable for multiple identical continuous measurement of characteristics of semiconductor wafers or similar products after all or selected stages of the manufacture thereof with the use of a generalized or universal sensor unit, which is based on the principles of the resonance sensor technology (RST) and can produce results of measurement in the form of a certain constantly recorded relative value.